any structural anomaly of the parallel, tubular, epithelial pits (crypts) with openings at the bases of the small intestinal villi; their thin walls are formed by columnar epithelial cells: mostly undifferentiated stem and intermediate cells and an increasing number of mucus-secreting goblet cells as the small intestine proceeds distally, all of which migrate out of the glands onto the villi, but also protein- (enzyme-) secreting Paneth cells that remain in the glands